1. FIeld of the Invention
The present invention relates to a plastic crate or tray, comprising a bottom and vertical side walls, said side walls being at least partially doublewalled, which plastic crate or tray comprises injection molding points.
2. Description of the Prior Art
In the prior art plastic crates or trays of the abovementioned type have been used. Said modern crates or trays comprising hollow corner columns and/or hollow side walls are also presently employed to a great extent as said plastic crates and/or trays may now be manufactured while economizing on the plastic material, while, nevertheless loading requirements are still met.
On injection-molding said plastic crates or trays comprising hollow corner columns and/or hollow side walls, problems will occur, due to the flow of injection-molded material into the hollows or openings of the relative mold.
These problems particularly involve uneven wall thicknesses in the corner columns or in the side walls, because of shifted cores, which may lead to an accelerated wear out or cleavage of the respective molds, whilst furthermore, flow lines will occur in the plastic crate or tray, which will disadvantageously influence the properties of strength of the respective crate or tray.
Actually, a plastic crate or tray is manufactured by injection-molding plastic into the lower side of the mold at the location of the bottom. Independent on the type of plastics, the type of injection molding apparatus and the preference of the relative laborer, a large central injection molding point is used, or several injection molding points being divided across the bottom.
When using a central injection molding point, the material flows through the space which will form the bottom of the plastic crate or tray toward the space which will form the side walls of the crate or tray, subsequently said material arrives in the space forming the side walls, thereby filling the space for the relative outer wall. When the core between the inner wall and the outer wall is higher, said core will be pressed outwardly, involving all the drawbacks mentioned hereinbefore.
Though several injection molding points may be used which are divided across the bottom, the hollows or openings of said points will then be filled simultaneously with the filling of the side wall mold hollows or openings.
In actual practice, however, this method of filling several spaces in a mold has been chosen, as the bottom forms the most loaded part of a crate or tray, so that said bottom should not possess any flowlines or seams and if this is the case as yet, said flowlines or seams should be sealed in a very early stage, thus avoiding any weak spots in said bottom.
Efforts have been made to obviate the shifting of the cores during the formation of the inner and outer wall of a double wall, by applying said core supports in the outer mold, which will result in gaps in the outer wall. It goes, however, without saying that said gaps are rather unattractive with a view to the properties of strength of the respective products, whilst the core supports may easily damage the relative molds in actual practice.